Ptolemy's Gate (3.5e Equipment)

From D&D Wiki

This legendary and forgotten item is a manuscript written by the great Ptolemies with the instruction to cast a spell like ability equivalent of an epic spell.
to cast this spell a character must sacrifice one 10th level spell slot & spend 2 month reading the manuscript. This item may also be used by a non spell caster class, but they must spend a whole year & 30,000 exp or give up 3 levels permanently to prepare the spell. The spell is cast with in a broken pentagram, and when the spell is cast the body remains while the soul ventures beyond the stars in to a realm of chaos. for every minute spent in there a character must make a fortitude or will save or loose 1 points of constitution permanently, if the save succeeds they instead gains a -1 penalty to their fortitude save permanently. In return for traveling to the nether world the caster forms a bond with a other worldly creature (chosen in joint by the DM and Player, usually one in which the Character has had contact with before). With this ability you gain the ability to summon a cohort with 10 HD for each round spent on the other place. This cohort gains the non-epic Pseuodonatural template for free.

Upon return the character's health is also reduced by 10% permanently, reduced by half on a successful will save followed by a fortitude save. The character is also Fatigued for one month, and shaken for one hour. A fortitude or will save (player's choice) removes the shaken condition.

The manuscript also contains notes on a possible additional ritual in which someone might bond to the outsider and share their bodies, sadly these notes are woefully incomplete and mostly hypothetical